Opportunities in Chemistry (1985) / Chapter Skim
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III. Control of Chemical Reactions
III. Control of Chemical Reactions
Pages 21-105
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From page 21... ...
A significant number of employed chemists are engaged in perfecting existing chemical processes and developing new ones. Their past success is attested by the vitality and strength of the U.S.
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From page 22... ...
· In enzyme catalysis, some characteristics of both heterogeneous and homogeneous catalysis appear. Whether natural or artificial enzymes are considered, large molecular structures are involved that can be seen to provide an "interface" upon which a dissolved reactant molecule can be immobilized, awaiting reaction (as in heterogeneous catalysis)
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From page 23... ...
Because of the unused bonding capability of the atoms at the surface, chemistry here can be qualitatively different from that of the same reactants brought together in solution or in the gas phase. However, when chemists are able to "see" what molecular structures are on the surface, our knowledge of reactions in conventional settings becomes applicable and opens the door to understanding and control of chemistry in this surface domain.
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From page 24... ...
As we learn more about molecular structures at the solid-gas interface (reactants, intermediates, and products) , a better understanding of surface chemical bonding will follow.
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From page 25... ...
The carbon-hydrogen bonds in normally unreactive hydrocarbons have been split by organorhodium, organorhenium, and organoiridium complexes. Hope for build-up of complex molecules from one-carbon molecules, such as carbon monoxide and carbon dioxide, is stimulated by recent demonstrations of carbon-carbon bond formation at metal centers bound in soluble metal-organic molecules.
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From page 26... ...
This is demonstrated by the iron example, which proves to be a functional unit in the ferrodoxin iron proteins that catalyze electron transfer reactions in biological systems. These cluster compounds, bound or "naked," furnish a natural bridge between homogeneous catalysis and bulk metal, heterogeneous catalysis.
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From page 28... ...
Most of the extraordinary instrumentation so far developed for surface science studies is applicable only at solid/vacuum interfaces. We need comparable capability at the solid/liquid boundary, and we will gain it from fundamental research in solid state chemistry, electrochemistry, surface analysis, and surface spectroscopy.
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From page 29... ...
Artificial-Enzyme Catalysis The most striking benefit of our expanding knowledge of reaction pathways and the analytical capacity of modern instrumentation has horn the develonment of our ability to deal with molecular systems ot extreme complexity. With the synthetic chemist's prowess and such diagnostic instruments as nuclear magnetic resonance, X-ray spectroscopy, and mass spectroscopy, we can now synthesize and control the structure of molecules that approach biological complexity.
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From page 30... ...
With benzene rings, enforced cavities have been made with the shapes of bowls, pots, saucers, and vases. We are clearly moving toward the next step, which is to build into these shaped cavities a catalytic binding site, such as a transition metal complex that is already known to have catalytic activity in Br Br p:Br Br air CAVITANDS WHAT SHAPE DO YOU NEED?
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From page 31... ...
It is an area ripe for development, and it deserves encouragement as a part of this program in chemical catalysis. Conclusion A significant share of our economy is built upon the chemical industry.
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From page 32... ...
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From page 34... ...
How chemistry has served us in the recent past and its potential role over the next 30 years will be considered for the following important energy sources and emerging alternatives: · Petroleum · Natural Gas · Coal, Lignite, Peat Petroleum Lou years. me s~gn~cance ~1~ ~ am: ~ _ 1 ~ _ 1 ~ · Shale Oil, Tar Sands · Biomass · Solar · Nuclear Fission · Nuclear Fusion · Conservation Petroleum use has increased steeply, worldwide: as much petroleum was taken from the ground between 1968 and 1978 as was produced in the preceding Ale ~ ^ of the increase is accentuated by the complex chemical processing requ~rect to convert the raw natural product into chemical forms that meet the demands of modern, high-compression engines.
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From page 35... ...
Then, by catalytic cracking, the large molecules are fragmented into lower boiling molecules. Alternatively, catalytic reforming can convert the molecular structures to higher octane forms.
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From page 36... ...
As these best fractions become depleted, we must learn to refine heavier crude oils (higher molecular weights) with lower hydrogen content and more undesirable contaminants, such as sulfur, nitrogen, and organometallic compounds.
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From page 37... ...
The combustion process is a tightly coupled system involving fluid flow, transport processes, energy transfer, and chemical kinetics. This complexity is epitomized in the methane-oxygen flame.
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From page 38... ...
However, again like petroleum, natural gas is limited in amount both worldwide and domestically and its nrod~.inn will undoubtedly peak one or two decades hence. Coal v, 7 —— ~— ~ _ ~ _ ~ ~ `, ^, ,, ,, ~ Coal is the most abundant of the fossil fuel energy sources.
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From page 39... ...
NCH3, N = 0 to 30 Mixed alcohol Cal to C3 oxygenated compounds Low-molecular-weight branched chains, hydrocarbons Ethylene glycol Fuel Fuels, via zeolite catalysts, chemical feedstock Feedstock for petroleum refiner~es Octane booster Chemical feedstocks High-octane fuel Polyester feedstock a The catalyst developed by Hans Fischer and Franz Tropsch in the early 1920s. The details of catalytic conversion of CO and H2 to particular desired products is an active research area.
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From page 40... ...
New extraction and upgrading processes are needed. Chemical kinetics, catalysis research, selective extraction, and selective absorption present directions worthy of study.
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From page 41... ...
To arrest this trend, we should be seeking new energy sources that do not release CO2. Solar energy is such an alternative.
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From page 42... ...
Thus natural photosynthesis is energized by near-infrared light through production of energy-storing intermediate substances with long enough lifetimes to await arrival of a second near-infrared photon. The second photon "stands on the shoulders" of the first so that their combined energy is adequate for making and breaking the conventional chemical bonds in the organic molecules of which plants are made.
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From page 43... ...
This observation has led to the idea that the photoreaction center consists of a pair of parallel chlorophyll rings held in close proximity by hydrogen bonding between · · ~ amino acid groups. Another promising approach to using solar energy is the direct conversion of sunlight to electrical or chemical energy with the aid of electrochemical devices.
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From page 44... ...
Whatever course society ultimately chooses, the minimization of its attendant risks will rely heavily upon the ingenuity of chemists and chemical engineers. Indeed, chemical research is essential to practically all phases of nuclear energy generation and the subsequent management of radioactive waste.
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From page 45... ...
; (2) to advance our knowledge of molecular dynamics (combustion)
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From page 46... ...
^ ^ ~ 7~ -am BRIM 97 t Or h~ ~~, ~ ~ ~ ..
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From page 47... ...
One of these, poly~para-phenylene terephthalamide3, has a tensile strength slightly higher than that of steel. But where this polymer really scores is in applications where the strength-to-weight ratio matters a lot, as it does in airplanes.
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From page 48... ...
By no PHYS I CS means does that exclude other disciplines. To make this point, we need only men tion the remarkable advances made in solid state physics over the last three decades characterizing and develop ing semiconductor materials.
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From page 49... ...
Recent additions to this arsenal include field flow fractionation, size exclusion chromatography, and highmolecular-weight mass spectrometry. By clever orchestration of these factors, chemists can design a polymer with tailored properties to build in plasticity or hardness, tensile strength, flexibility, or elasticity, thermal softening or thermal stability, chemical inertness or solubility, attraction or repulsion of solvents (wetting properties)
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From page 50... ...
The elasticity in the polymer chain direction can now be calculated from 1,600,000 bond lengths, bond angles, 2430,000 and the experimental defor mation force constants de rived from infrared spectro scopic measurements. The 2,326,000 resulting progress Is ev~ g20,000 denced in Table ITI-5 which compares demonstrated ten sile strengths of two organic 500,000 polymer fibers to both alumi num alloy and drawn steel.
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From page 51... ...
The impact of such research advances will be significantly felt in our international economic and strategic posture. TABLE III-5 Polymer Fibers Compete as Structural Materials Tensile Strengtha Tensile Strength per Unit Weighta (1.0)
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From page 52... ...
properties. It is just this control that lies behind the production of fibers of exceptionally high tensile strength, which, because of their better strength-to-weight ratio, can replace steel in products ranging from airframe construction to bulletproof vests.
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From page 53... ...
The diffraction effects can be reduced by using shorter wavelength radiation, and chemists are now at work developing new organic polymers sensitive to exposure to electron beams, X-rays, and short wavelength ultraviolet light.
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From page 54... ...
Instead of a pulse of electrons in a copper wire, a pulse of light is sent through the transparent fiber to convey a bit of information. The critical development that made this optical technology possible was the production of highly transparent silica fibers through a new process known as chemical vapor deposition (CVD)
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From page 55... ...
Novel Electrical Conductors Semiconductors The modern age of solids was launched during the 1950s by brilliant advances of solid state physicists as they developed deep understanding of pure semiconductor materials. There were early challenges to chemists, too, as it became clear that elemental silicon and germanium were needed in single crystal form with impurity levels as low as one part per 100 million.
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From page 56... ...
These two roles are filled, respectively, by TTF and TCNQ. The surprise is that this charge transfer between two neighbors in the crystalline stack provides a mechanism for current flow up and down the stack, the length of the crystal.
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From page 57... ...
Because response to light can be designed to match the solar spectrum, these polymers give us hope for cheap organic photovoltaic cells with which to convert solar energy to electricity. Extensive research is in progress to develop lightweight, high power density, rechargeable batteries with polymeric electrodes.
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From page 58... ...
Solid State ionic Conductors Solid materials with ionic structures are now known with ionic charge mobilities approaching those in liquids. Investigations of such materials over the last decade have already led to their use in memory devices, display devices, chemical sensors, and as electrolytes and electrodes in batteries.
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From page 59... ...
The performance of the Tow-cost solar cells made of such material depends critically upon hydrogen impurities chemically bound to the interstitial "dangling bonds" of the silicon atoms randomly lodged in the amorphous solid. Also, inorganic nonmetallic glasses are important for optical fiber communication and for packaging solid state circuits.
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From page 60... ...
. Such a transient high temperature pulse can cause significant chemical and physical changes, modifying the surface, forming surface alloys, and promoting specific chemical reactions when coupled with vapor deposition.
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From page 61... ...
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From page 62... ...
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From page 63... ...
They account for the recent acceleration of progress that gives chemistry unusual promise. Three of these intellectual frontiers of particular promise will be explored here: molecular dynamics, some aspects of chemical catalysis, and reaction pathways.
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From page 64... ...
At this time there are probably fewer than 500 pulsed and tunable dye lasers being used for nanosecond spectroscopy throughout the world. There may be fewer than two dozen laboratories studying chemical reactions in the 1()
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From page 65... ...
Some of the phenomena to follow also depend upon use of short-puIse laser excitation, molecular beams, fast-response electronic circuits, and computer-controlled data acquisition and interpretation. Such instrumentation has opened new frontiers of chemistry.
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From page 66... ...
Light emitted in the first few picosecon(ls shows that the energy has not yet left the benzene unit where it was absorbed. The time scale for appearance of vibrational excitation in the alkyl tail (leFL U ORES CEN CE REVEALS I NTRAMOLECULAR VIBRATIONAL RED ISTRIBUTION 6~>>~7 COLD RING HOT TAIL .
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From page 67... ...
Of course, a reaction between two molecules requires that they come close together, so chemists must also know about energy transfer caused when molecules collide. Some experiments successfully explore these processes with ·1 ~ · 1 · ~ ~ · · ~ ~ ~ ~ ~ ~ · ~ ~ · ~ ~ vibrational excitation using a short-pulse laser and time-resolved infrared fluorescence measurements.
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From page 68... ...
The data encouraged ab initio and trajectory calculations to investigate the de"Molecular beams" move even closer toward "state-to-state" investigations. In such experiments, reactants meet at such low pressures—10- if atmospheres that each reactant molecule has at most one collisional opportunity to react, and the products have none.
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From page 69... ...
Despite the difficulty and cost of molecular beam experiments, the "state-tostate" information is of crucial relevance to many of the fundamental questions of molecular dynamics and to practical problems of combustion. The molecular beam technique will increase in importance as the sophisticated equipment becomes more generally available.
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From page 70... ...
QUASI CONTINUUM DISSOCIATIVE CONTINUUM ISOTOPE SEPARATION THROUGH MULTIPHOTON EXCITATION n* SO ~ F 5 This behavior has already added substantially to our understanding of infrared radiation, and it helped trigger a host of studies on internal energy flow within excited polyatomic molecules.
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From page 71... ...
This gaseous compound gave the first convincing evidence that multiple photon excitation really occurred so rapidly that collisional energy transfer could be avoided. The successful use of SF6 for sulfur isotope separation could have heavy implications for human history.
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From page 72... ...
. These clues may indicate that supersonic jet reactant cooling in molecular beam experiments will be productive.
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From page 73... ...
. It is of theoretical importance because so much is known experimentally about the product energy distribution that the F + H2 reaction has become a prototype to test and develop our understandings.
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From page 74... ...
Theorists now aspire to extend their calculations to compounds involving elements deeper in the Periodic Table. These are important chemical species in catalysis, isotope separation, organic conductors, and biological mechanisms.
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From page 75... ...
, chemical processing, and photochemical reactions will all benefit from better theoretical models. Bulk reactions can be understood and controlled only with microscopic understanding of the dynamical details, including energy transfer processes.
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From page 76... ...
Then, if 10 micrograms or more of a crystalline material become available, every sterochemical nuance of structure is displayed through X-ray spectroscopy. Physical organic chemistry is the second major area; it seeks to relate changes in physical, chemical, and spectroscopic behavior of organic compounds to changes in molecular structure.
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From page 77... ...
A third dimension discussed later in this chapter is the use of "tuned photochemistry" in which selectivity can be increased by use of lasers to induce photochemical reactions. Striking developments have occurred in the use of catalysis to reach new synthetic Coals.
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From page 78... ...
Of course, a left-handed glove will not fit a right hand, so it cannot serve the function of a right-handed glove. It is the same in the function of biolo~in~llv _ A ~ ~ 1 ~ I 1 ~ ~ J Important natural proctucts where this "handedness" aspect of molecular structure assumes critical importance.
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From page 79... ...
Inorganic Chemistry There is great intellectual ferment now in inorganic chemistry, much of it at the interfaces with sister disciplines: organometallic chemistry, bioinorganic chemistry, solid state chemistry, biogeochemistry, and other overlapping fields as yet unnamed. The same instrumental array that is so effective in organic
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From page 80... ...
Answers to these significant structural questions are being sought by spectroscopists, synthetic chemists, and crystallographers around the world who want to learn how Nature has solved these extremely complicated chemical problems. These scientists are joined by those studying reaction dynamics in these systems, seeking to describe the details of electron transfer through protein systems, energy transfer and utilization in cells, electrical signal transfer through nerves and across gaps, and molecular recognition at all levels.
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From page 81... ...
Bridging the gap now between solid state chemistry and the chemistry of simple substances in solution is the field of metal cluster chemistry. The field has grown from a subfield of organo-metallic chemistry itself an interface between traditional areas to a discipline of its own.
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From page 82... ...
~ .. 11 3 47A o VAN DER WAALS MOLECULESTHE WEAK lNTERACTlONS THAT GOVERN SOLUBILlTY, GAS lMPERFECTlON, LIQU EFACT1ON ~ Ar ~ - ~ ~ cryogenic matrices and under molecular beam conditions using supersonic jet cooling.
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From page 83... ...
In fact, the entire array of tools of the physical organic chemist is now being used in the stub of the reactions of or~nom~t~llir `;umpounas Elan results as surprising and as useful as when they were first applied in organic chemistry. The chemistry of the main group elements those to the right of the Periodic Table has in the past largely been explored abroad.
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From page 84... ...
New tools for characterization of these compounds are being developed—t70 and metal nuclide NMR, chiral supports for chromatographic analysis- that will help the experimentalist establish the conceptual framework of the field. Already a metal-containing, highly enantioselective system for the epoxidation of prochiral allylic alcohols has been discovered and assimilated into the organic chemist's battery of synthetic techniques.
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From page 85... ...
III-D. INTELLECTUAL FRONTIERS single amine substituent to _ adamantane gave adaman tine (l-amino-adamantane)
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From page 86... ...
Important theoretical developments have accelerated progress here. Electron transfer theories from inorganic chemistry and charge-transfer views on organic systems have been unified to provide a basis for predicting oxidation
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From page 87... ...
Many natural products and complex molecules of medicinal importance involve high energy ("strained") molecular structures.
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From page 88... ...
bond angles around carbon. Thus, rings containing three or four carbon atoms are relatively unstable and, hence, difficult to synthesize; they were long sought just because they were chemical oddities.
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From page 89... ...
Now we are seeing a broader resurgence of interest in the chemistry and properties of novel solids. Advances in both experimental techniques and chemical understandings are permitting more systematic strategies that actively draw upon the established principles of chemical thermodynamics, chemical kinetics, chemical bonding, and molecular structure.
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From page 90... ...
so CONTROL OF CHEMICAL REACTIONS
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From page 91... ...
Lasers An array of complementary experimental methods has come into existence that makes molecular dynamics one of the most active frontiers of chemistry research. On this frontier, perhaps the highest potential for gain lies in understanding, at last, the factors that control temporal aspects of chemical change.
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From page 92... ...
Tunable lasers can now be conveniently operated over the wavelength range from 1600 in the vacuum ultraviolet to 4 microns in the infrared. Already in the state-of-the-art stage are lasers that extend the wavelength range to beyond 20 microns in the infrared and to less than 1000 A in the vacuum ultraviolet.
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From page 93... ...
As discussed later in this section, these devices currently operate in the TR but will soon be extended into the UV and vacuum ultraviolet regions. Finally, a combination of excimer lasers, dye lasers, and nonlinear mixing techniques will be used to generate coherent soft X-rays in an intensity range intermediate to the free electron laser and the synchrotron, but at a cost considerably less than either.
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From page 94... ...
These ancillary pieces of equipment will add at least $20K to $40K to the laser cost. Consequently, the cost of equipping a laboratory with tunable sources over a wide spectral range with both pulsed and continuous capability can easily exceed $250K.
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From page 95... ...
Several picosecond ki- Few Fast electronic state re- High ~ Mode-locked netics taxation rep. ~ pump—1 or 2 Decay of coherent pro- rate J Synchronously pumped cesses in the condensed dye laser 6, 14 phase High l Mode-locked power J Solid state 3, 14 C
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From page 96... ...
Because of the success of these instruments, the Fourier transform algorithm is now being ; ~ ~~ ~~ ~ ~ ~ 1 : ~ ~ ~ ~ 1 ~ ~1~ ~ ~1~ ~ _ 1 ~ _ · ~ 1 . lil~Vlp~l=~U lilLU ~l~-~il~lnl~l, microwave, ion cyclotron resonance, dielectric, and solid state NMR equipment.
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From page 97... ...
Most dynamical simulation procedures applicable to biological molecules require energy solutions for the simultaneous motions of many atoms. A conventional 100-picosecond molecular dynamics simulation of a small protein in water would require about 100 hours on a DEC VAX 11/780 or 10 hours on an IBM 3033.
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From page 98... ...
for the investigation of elementary chemical reactions, energy transfer processes, and intermolecular potentials. Capabilities A typical, crossed molecular beam apparatus can contain as many as eight differentially pumped regions provided by various high-speed and ultrahigh vacuum pumping equipment.
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From page 99... ...
Costs A state-of-the-art crossed molecular beam apparatus equipped with highintensity beam sources and data acquisition electronics will cost $350K to construct and $40K/year to maintain. If a tunable laser is also used for the excitation of reagent atoms or molecules to specific quantum states, an additional $100K will be needed.
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From page 100... ...
Such a quantum jump in brightness will surely lead to new types of experiments. For many chemical applications, the intensity will be of particular importance if the design specifications attempt to optimize radiation in the vacuum ultraviolet spectral region rather than regard it as a parasitic use.
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From page 101... ...
6-GeV synchrotron light source (which would find some limited use in chemical applications) has been priced at about $160M for construction, including its proposed 10 insertion devices and associated beam lines.
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From page 102... ...
Softer X-ray synchrotron light sources, for which there may be more general chemical applicability, might cost in the range $60M to $80M. At the storage rings at the University of Wisconsin ancI at the Brookhaven NSL:both clesignec!
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From page 103... ...
Again, a variety of novel chemical applications could be explored in photochemistry and fast chemical kinetics, as well as multiple photon and other nonlinear processes. Instrument developers should be aware that most of these applications could be productively pursued in the visible and normal ultraviolet spectral ranges.
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From page 104... ...
— : : : : ~ (AH: iMISTRY:isacentral~science~ i: ~ ~ i: : : ~ : : : : : A : :: : : If: ~ ~~:~:~:tnat~re~sponusto~socl~tal~needs.
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From page 105... ...
. One method used, nuclear magnetic resonance (NMR)
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